1. Field of the Art
The present invention relates to a process and apparatus for producing a dysprosium-iron alloy and a neodymium-dysprosium-iron alloy. More particularly, it relates to a process of continuously producing a dysprosium-iron alloy of high dysprosium content and a neodymium-dysprosium-iron alloy of high dysprosium and neodymium content, both of which can be advantageously used as an alloying material for producing a high-quality permanent magnet of rare earth metal, iron and boron, because of their freedom from harmful impurities and non-metallic inclusions.
2. Related Art Statement
Dysprosium (Dy) is advantageously used as an alloying material for a recently developed high-quality permanent magnet made of neodymium, dysprosium, iron and boron, for the purpose of increasing its magnetic coercive force (a Japanese laid-open patent application: TOKU-KAI-SHO-60 No. (1985)-32306 can be referred). It is therefore expected that the demand for dysprosium will be increased in future. Although metallic dysprosium can be added to the magnet with some effect, a dysprosium-iron alloy is preferable to metallic dysprosium, in respect of handling for addition to the magnet, since metallic dysprosium has a comparatively high melting point, 1409.degree. C. Further, a dysprosium-transition metal (e.g., iron) alloy is now under review, for use as a material for magnetooptical disks.
A neodymium-dysprosium-iron alloy is advantageous for use as the alloying material for such a permanent magnet, if the composition of the alloy has a constant ratio of neodymium to dysprosium. Since neodymium and dysprosium are simultaneously introduced into the permanent magnet by use of the neodymium-dysprosium-iron alloy, the manufacturing cost of the permanent magnet is decreased.
Four processes, which are commonly known in the art, of manufacturing an alloy of a rare earth metal and a high-melting-point metal are described below. All of them can, however, not be satisfactory, because of containing some inherent disadvantages or problems, as the practical and industrial process operable continuously.
(A) A method wherein rare earth metal or its alloy is prepared beforehand by means of electrowinning the same in a bath of electrolyte or by means of reducing a rare earth compound with an active metal, and the obtained rare earth or its alloy is melted together with another metal for alloying them:
The method, however, is problematical in the first step of preparing the rare earth or its alloy. In the electrowinning method, two techniques can be named as a prior art: Electrolysis is an electrolyte bath of fused chlorides (raw materials), and electrolysis of rare earth oxide (raw material) dissolved in an electrolyte bath of fused fluorides. The former technique suffers a problem of a difficult handling of the fused chlorides, and a further problem resulting from the batch style which is not suitable for a continuous operation in a large scale. On the other hand, the latter technique has a problem of a low solubility of the oxide in the electrolyte bath, which hinders a continuous electrolysis operation and results in an accumulation of sludge on the bottom of the electrowinning cell. It is recommended for a continuous and large scale production that the rare earth of its alloy is produced in a liquid state, but it is impractical to raise to an excessively high electrolysis temperatures at which the electrolysis operation is conducted, according to a high melting point of the rare earth to be obtained, since at higher temperatures non-metallic inclusions more easilly enter into the rare earth or its alloy produced.
On the other hand, the reduction method utilizing an active metal belongs to a batch system and is therefore not suitable for a continuous and large scale production. Further, this method has a disadvantage of use of an expensive active metal (reducing agent) and use of expensive materials for the exclusive apparatus. This method has another disadvantage of involving an additional step for removing the residual active agent.
(B) Another method wherein alloying is executed by means of reducing a mixture of a rare earth compound and a compound of metal to be alloyed with the rare earth by utilizing a reducing agent (e.g., calcium hydride for a Sm-Co alloy):
This method needs an expensive reducing agent, and can not be, either, an exception of the batch-style method, being unsuitable for a continuous and large scale operation.
(C) Still another method wherein an alloy of rare earth and a metal to be alloyed with the rare earth is electrodeposited on the cathode by electrolytic reduction which is carried out in a bath of electrolyte dissolving both a compound of the rare earth and a compound of the metal to be alloyed with the rare earth (a U.S. Pat. No. 3,298,935 can be referred), therein:
This method is problematical in that it is difficult to keep the chemical composition of the alloy produced on the cathode, uniform over a long period of time during the electrolysis operation. Further, in the case where oxide is used as a raw material, the method has a problem of a low solubility of the oxide in the electrolyte bath, which hinders a continuous electrolysis operation.
(D) So-called consumable cathode method, wherein rare earth is electrodeposited by electrolytic reduction on a consumable cathode of a metal and alloyed with the metal of the cathode, in one step which is executed in a suitable bath of electrolyte of fused salts (can be referred "U.S. Bur. of Min., Rep. of Invest.", No. 7146, 1968, and Japanese Pat. Nos. 837401 and 967389):
The shortcomings will be described below: in the case where a rare earth oxide is used as a raw material to be reduced, the method suffers problems, as stated previously, of a low solubility of the rare earth oxide in the selected electrolyte bath and of an accumulated sludge of the oxide; moreover, conducting the electrolysis operation at increased temperatures for overcoming those problems results in producing a deteriorated alloy containing an increased amount of non-metallic inclusions as coming from the structural materials of the electrowinning cell. Further, the recovery of the produced alloy is carried out in a batch style which is unsuitable for a continuous and large-scaled operation.
Some of the present inventors and others have file a U.S. patent application (Ser. No. 776,800, Sep. 17, 1985) in which a process of producing neodymium-iron alloy and an apparatus therefor are described and claimed, but no disclosure is found about a process of producing dysprocium-iron alloy or neodymium-dysprosium-iron alloy nor an apparatus therefor.
Metallic dysprosium has been, in fact, almost useless, and the industrial manufacturing process of getting the same has not been settled, yet, except for the above-mentioned reduction method (A) in which dysprosium can be produced in a small quantity. It can be said that no industrially practical process is firmly established for continuously producing dysprosium. It is a matter of fact that there has been not established an industrial method of continuously producing a dysprosium-iron alloy for use as an alloying material for a high-quality permanent magnet made of neodymium, dysprosium, iron and boron. Similarly, there has been no established method of continuously producing a neodymium-dysprosium-iron alloy for the same use.